A Type I / Type 1 ambulance is mounted on a truck-style chassis. The driver compartment resembles a pickup truck. The connection between the cab and patient module is a small pass-through that looks like a window. 

A Type III / Type 3 ambulance is mounted on a cutaway van chassis. The cab is an integral part of the ambulance unit. The connection between the cab and patient module can vary in appearance, but generally looks more like a doorway than a window. This allows for easier transfer of items from the patient compartment to the driver compartment and vice-versa.